The present invention relates to the exploration and production of petroleum from earth formations, and more particularly to methods for determining the amount of oil present in such a formation.
In the petroleum industry, one of the most valuable and informative techniques for determining the characteristics of an earth formation located well below the surface, and the nature of the fluids which it may contain, is to remove and bring a portion of the formation to the surface for analysis. This is most commonly done by "coring" the formation. Of course, physical conditions in the formation are substantially different from those at the surface: pressures and temperatures are ordinarily enormously elevated over surface conditions. Therefore, fluids and gases present in porous rock samples very often evolve from those samples as they are recovered from the formation. To the extent that such liquids and gases are lost, the accuracy of the evaluation of the formation production potential is accordingly impaired.
To control this problem, a technique called "pressure coring" is often employed. With pressure coring, the core is contained at substantially its original formation pressure until a proper analysis can be made. Pressure coring, while overcoming such fluid loss problems to a great extent, is quite expensive (for example, $100,000-$200,000 for a single coring operation)
Recently, a technique called "sponge coring" has been developed. It is substantially less expensive than pressure coring. In sponge coring, the core is surrounded by a polyurethane sponge lining on the inner barrel of a conventional core barrel. As the core is brought to the surface, any oil that bleeds from the core (and this can be as much as 50% of the original core fluid) is caught and retained by the sponge liner. An analysis is then made of the fluids (oil and water) captured by the sponge, and the analysis of the core adjusted accordingly.
The importance of coring in the production of petroleum has recently been increasing as more and more secondary and tertiary recovery is being made of petroleum reserves. In a formation undergoing primary production, the original reservoir fluids are little altered from their condition for the last several thousand years. They may migrate as the oil is produced, but their properties are little changed. However, when fluids and/or other compounds are injected into a formation to stimulate its production, the nature of the connate fluids is accordingly altered, sometimes to a very substantial extent. When this occurs, the more traditional well bore logging tools may be unable to provide further useful information. In all too many instances, the only way to determine how much oil is left, and thus whether it can be produced economically, is to go down there and take a core sample.
It will therefore be appreciated that the analysis of the oil content of the core sample can be critically important. The final true residual oil saturation of a formation is a determination that can make or break a multi-million dollar enhanced recovery project. Hence, the considerable interest in sponge coring, which can cost as little as only 20-30% as much as pressure coring.
Currently, a major problem with sponge coring is that, curiously, the industry has not been able to make consistently reliable measurements of the amount of oil in the sponge, to the degree of accuracy necessary for meaningful reservoir analysis. Many service companies have tried, and are continuing to try, to measure the oil in the sponge either by extracting the oil with solvents or squeezing the sponge in a vice, or both. However done, usually not all the oil is removed, while often some of the sponge is dissolved instead. Thus, unextracted oil is not counted while extracted sponge is erroneously counted as oil. This is demonstrated, for example, in FIGS. 2, 3, and 4, which are existing (prior art) results of blind tests sent to several different service companies. In these tests, sponge samples were prepared by saturating first with deionized water and then spiking with a known volume of mineral oil. The results reported back by the service companies graphically illustrate the problems with current technology.
For example, in FIG. 2, a three-step mechanical solvent extraction technique was employed. Heat was used in the first step, followed by the administration of dichloromethane solvent, then repeated squeezing and draining in a hydraulic press. The solvent was then driven off.
In FIG. 3, two different methods were used, one being a two-step process involving a mechanical solvent extraction of the sponge and distillation of the extracted fluid, and the other being a two-step process involving mechanical extraction of the sponge followed by retorting the sponge. Since no apparent systematic differences were noted, data for both methods are combined in FIG. 3.
FIG. 4 illustrates the results of a process in which the sponge is first placed in a steel container with dichloromethane solvent, and then repeatedly compressed with the addition of more solvent until the drained solvent is clear. The solvent with oil is then separated from water and the solvent subsequently removed. The results at this point are then corrected by repeating the process with a sponge spiked with a known weight of oil and comparing the results.
As can be seen from the drawings, none of these techniques yields an accurate or precise determination of the injected oil volumes. Accordingly, a substantial need still remains for a method for extracting oil from the sponge of a sponge coring operation which will accurately reflect the actual amount of oil captured by the sponge, leaving the sponge itself substantially unaffected. Preferably, a solvent should be found which will be gentle on the polyurethane sponge, neither swelling nor dissolving it, and which will be a good solvent for all the components of crude oil, including heavy resins, waxes, and asphaltenes. Preferably, the solvent should have a low boiling point so that it can be simply evaporated or boiled off to leave only the volume of crude oil which was contained in the sponge. Alternatively, a rapid and simple analytical technique for the solution of solvent and oil should be provided so that the volume of the oil which was captured by the sponge can be readily and rapidly determined.